It took just five years for glyphosate to become almost useless for controlling the weed Palmer amaranth in sections of the US soybean and cotton industries.
Dr Jason Norsworthy of the University of Arkansas in the US said the first resistant Palmer amaranth was found in 2005 and by 2011 more than 60 per cent of soybean crops and 87 per cent of cotton crops had glyphosate-resistant Palmer amaranth.
He said this showed that the technology had effectively been completely lost.
Where Palmer amaranth had developed resistance to glyphosate, paddocks had to be abandoned and in some instances people had lost their farms, Dr Norsworthy said.
He said the resistance issue had also now seriously compromised conservation tillage: “For example, in Tennessee, the use of no-tillage has gone from 85 per cent to 50 per cent.”
Reduced rates were also applied to save money.
Many growers also delayed the application of glyphosate to kill as many weeds as possible with a single application.
Like all researchers, Dr Norsworthy points to the near total reliance placed on glyphosate: “Three to four years ago 98 per cent of our cotton, soybean and corn acreage was Roundup Ready®. It was a prescription for disaster.”
Reduced rates were also applied to save money. Many growers also delayed the application of glyphosate to kill as many weeds as possible with a single application.
Looking back, Dr Norsworthy said cropping systems inthe US lacked diversity. “In 1995 there were 19 different herbicides used across five per cent of the soybean area in the US. By 2005 the only in-crop herbicide used on soybeans was glyphosate.”
Growers in Arkansas are now using glufosinate-resistant (Liberty Link®) soybeans to control glyphosate-resistant Palmer amaranth. Unfortunately, about a third of glufosinate-resistant soybeans are being treated with nothing but glufosinate.
This aside, Dr Norsworthy said US growers were keenly aware of the issue and many were now paying the US Department of Agriculture to develop herbicide-resistance management plans for them.
Among the management tactics being encouraged are the use of full herbicide label rates, reduced row spacings to increase crop competition and to reduce in-crop temperature, the use of cover crops, narrow windrow burning and mouldboard ploughing where appropriate.
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